Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus, and more specifically to a configuration for correcting a skew of a sheet.
Description of the Related Art
Conventionally, an image forming apparatus such as a printer, a copier, and a facsimile includes a skew correcting portion conveying a sheet to an image forming portion and correcting a skew of the sheet. As such skew correcting portion, Japanese Patent No. 2893540 proposes an apparatus using a registration roller pair correcting a skew of a sheet and aligning an image formed in the image forming portion with a front end of the sheet. In correcting a skew of the sheet, firstly, the front end of the sheet being conveyed by an upstream conveying roller pair is caused to abut against a nip portion of the registration roller pair being at a stop.
Thereby, the sheet deflects and forms a loop. The front end of the sheet then follows a nip line of the registration roller pair due to stiffness of the sheet, so that the skew of the sheet is corrected. It is noted that after the correction, the image formed in the image forming portion is aligned with the front end of the sheet by adjusting timing for starting to rotate the registration roller pair.
Here, conventionally, there is an apparatus in which a registration roller pair is disposed along a curved conveying path and a loop space is provided so that a loop of the sheet can be formed upstream in the sheet conveying direction of the registration roller pair. Normally, in a case where a conveying path composed of a pair of conveyance guides is curved, the sheet abuts against the registration roller pair in a state partially separated from an outer conveyance guide in the curve direction by stiffness of the sheet itself and then forms a loop in the loop space.
However, because size of the loop formed by the sheet is different depending on the sheet being conveyed, there is a case where an enough loop space cannot be obtained and a skew cannot be corrected.
Then, conventionally, Japanese patent Application Laid-open No. Hei. 8-113389 proposes a sheet conveying apparatus in which a part of the conveyance guide guiding a sheet to the registration roller pair is configured to be swingable. Then, a loop forming area is assured by temporarily receding the conveyance guide, swingable by the loop of the sheet (referred to a ‘swingable guide’ hereinafter), from the conveying path. The sheet conveying apparatus corrects a skew of the sheet by forming a loop corresponding to a loop amount of the sheet in that area.
However, the sheet conveying apparatus described in Japanese patent Application Laid-open No. Hei. 8-113389 is unable to set a swing force of the swingable guide that enables to form similar loops enabling to correct a skew for both of a non-stiff sheet and a stiff sheet such as a thick sheet and a coated sheet. That is, if the swing force of the swingable guide is set so as to be able to form a loop by the non-stiff sheet, the swingable guide considerably swings by being pressed by a front end part of the sheet before the sheet abuts against the registration roller pair. In such a case, there is a possibility that the sheet deviates out of the conveying path, causing conveyance failures such as jamming and troubles such as scratch and fold.
Still further, if the swing force of the swingable guide is set to be higher, while it allows the stiff sheet to form an adequate loop and to correct a skew, the non-stiff sheet is unable to swing the swingable guide. In this case, because the sheet cannot form an adequate loop, a skew correcting ability drops.